Planetary gear sets have a sun gear, a ring or internal gear and a plurality of pinion, or planet, gears which mesh with the sun and ring gears. The planet gears are rotatably supported from a carrier. The planet gears are often subjected to thrust loads that are imposed by virtue of a slight difference in the angular disposition of the helical teeth on either the sun or ring gears or between the pinion gears and the sun and ring gears. Some thrust loads are imposed by the shafting arrangement in the transmission. For example, thrust loads can be imposed as a result of axial movement of the output shaft. Thrust loads are transmitted between the planet gears and the supporting carrier structure through thrust washer bearing means which permit relative rotation during transmission of the thrust loading.
The planet gears are generally mounted on bushings, or needle bearings. If needle bearings are present, thrust forces can be generated within the needle bearings, and those thrust forces must also be transmitted to the carrier by which the planet gears are supported. The needle bearings not only rotate on their longitudinal axis, they also planetate about the spindle upon which they are operatively mounted rotatably to support a planet gear. For that reason the angular velocity at which the needle bearings planetate is different from the rotational speed of the associated planet gear on the spindle. Thus, when a single thrust bearing is used, it must absorb the thrust forces while simultaneously being subjected to the relative speed differences between the gear, the needles and the carrier.